1. Field of the Invention
The invention is based on a method for producing at least two transformers, especially for distribution systems, in which solid insulation of the transformers is dried before the addition of insulating means, in that the low-voltage windings in the transformers are short-circuited, and the high-voltage windings are connected in parallel with one another and are heated by means of a low-frequency AC current, and in this case the mean temperature of the windings of the parallel-connected transformers is monitored by measuring the low-frequency AC current.
2. Discussion of Background
In the production of transformers which are provided especially for power distribution tasks, it is necessary for solid insulation of the windings to be dried first, before filling of the insulating means, such as insulating oil, which surround the high-voltage and low-voltage windings. According to a method which is disclosed in DE 3,927,964 A, this drying can be achieved, for example, by the low-voltage windings of the transformers being short-circuited and by the transformers being heated by means of a low-frequency AC current which is supplied to the high-voltage windings. In order in this case to make it possible for the method to be carried out quickly and hence economically, and, at the same time, in order to avoid an excessive heating voltage (causing undesired electrical flashovers on the windings), the short-circuit impedance in the heating circuit is kept low by reducing the frequency of the heating current. Normally, when the method is being carried out, the transformers which are enclosed in a vacuum autoclave under atmospheric conditions or with a slightly reduced pressure are first heated to a mean winding temperature of approximately 100.degree. C., using low-frequency AC current. This temperature is monitored indirectly by measuring the heating current. The temperature is kept constant by suitable adaptation of the heating current. In this case, the solid insulation is predried. The transformers are thus completely dried at approximately 130.degree. C., in a variable vacuum and with the residual moisture being emitted. This vacuum can subsequently be used for the complete and rapid filling of the transformers with the insulating means.
However, in order to heat transformers of different type and size as uniformly as possible, this method requires suitably adapted power supplies. Such power supplies comprise in each case at least one frequency converter and control systems, and are comparatively costly. In addition, even slight deviations of specific method parameters can adversely affect uniform heating and hence the drying of the solid insulation. Furthermore, during the drying of a plurality of parallel-connected transformers, only the mean temperature of the high-voltage and/or low-voltage windings of all the transformers can be determined.